Pencil comprising a marking core and a porous resin sheath

ABSTRACT

Pencils made with a resin-based sheath to replace wood sheaths. A sheath material consisting essentially of a resin binder, a fibrous filler and a metallic soap is extruded around a marking core. The resulting extrudate is immediately chilled after leaving the die, and cut into pencil lengths. The apparatus includes core feeding and transport means, core preheating means and chilling means. The resulting pencils possess the physical qualities associated with wood-sheathed pencils due to the fine, closed cell structure of the sheath material. The surface of the sheath is smooth and may be coated with a pigmented resin or painted.

This application is a division of application Ser. No. 348,664; filedApr. 6, 1973, now U.S. Pat. No. 3,875,088; Apr. 1, 1975, whichapplication is a Continuation-In-Part of application Ser. No. 163,303;filed July 16, 1971, now abandoned.

The invention relates to pencil manufacture, and is more particularlydirected to the manufacture of pencils having an extruded, syntheticresin-based sheath.

BACKGROUND OF THE INVENTION

Pencils are normally formed by enclosing the marking core in woodensheaths. The wood used must meet certain physical standards of strength,dimensional stability and sharpenability. The present commercial processfor making pencils is a multistep one, time consuming and relativelyexpensive. Substitutes for the wooden sheaths as well as for theinvolved manufacturing process have long been sought. It is, however,difficult to form a sheath material which is satisfactorily bonded tothe marking core, and which the user will accept as a substitute for thewell-known wood sheath. This is due to the fact that the sheath mustmeet certain desired criteria including sharpenability, adequatestiffness, good flexural or breaking strength, relatively low densityand bondability to the marking core.

Many attempts have been made to manufacture pencil sheaths from amaterial other than wood. It has been proposed to manufacture pencilsheaths by extrusion and subsequent drying of an aqueous pulp of wood orpaper with a suitable binder, or to tightly wrap the marking core withpaper and the like. In the first instance, such attempts have beenunsuccessful because the necessity of expelling large amounts of waterfrom the extrudate gives rise to warped pencils and loose cores. Wherepencils have been made by wrapping the core, problems have arisen inadhering the wrapping material to the core, as well as adhering thelayers of wrapping material to each other, and in producing a straight,properly aligned sheath that could be readily sharpened by a penknife ormechanical sharpening device.

U.S. Pat. Nos. 2,988,784 and 2,790,202 disclose a sheath composition, amethod, and an apparatus for extruding a sheath around a marking core tomake a so-called scissible writing instrument. The sheath compositiondisclosed is a mixture of a main ingredient (wood flour and the like), awaxy substance such as chlorinated naphthalene, a thermoplastic bindersuch as polyvinyl chloride, a plasticizer for the binder, and alubricant. The sheath composition is blended and used in a completelydry condition, this being the patentee's way of avoiding thedifficulties encountered in the prior art compositions and methods.

In the method and apparatus of U.S. Pat. No. 2,790,202, techniques andmeans are used to extrude the sheath around leads fed along avertically-oriented axis. Cooling of the sheath is accomplished beforethe sheathed lead leaves the die nozzle.

The composition, method and apparatus of U.S. Pat. Nos. 2,988,784 and2,790,202 present certain inherent difficulties and limitations. Amongsuch difficulties are the need for working with completely dry sheathmaterial, the need for using an adhesive to bond the core to the sheath,and the necessity to accurately control the radial component of pressurein relation to the axial component of pressure on the sheath material toovercome the back pressure in the die brought about through the verticalfeeding of abutting core lengths. To the best of applicants' knowledge,this prior art teaching which overcomes some of the disadvantages andlimitations of the art preceding it has a number of limitations of itsown, which have apparently prevented it from achieving any commercialsuccess.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a novel and improvedcomposition which can be extruded around a marking core to form thesheath of a pencil. It is another object to provide a composition whichmay be extruded at temperatures lower than usually employed, and whichis directly bondable to the marking core, thereby eliminating the needfor an adhesive. It is still another object to provide such a sheathcomposition which when extruded and cooled is made up of fine cellsimparting to the sheath good sharpenability and a smooth, denser outersurface which readily lends itself to the application of a coloredcoating.

It is another object of the invention to provide an improved pencil thesheath of which is formed of a thermoplastic material and possesses thecharacteristics of sharpenability, adequate stiffness, adequate flexuralor breaking strength, low density, and bondability to the marking coreassociated with wood-sheathed pencils.

It is a further object of this invention to provide an improved methodof forming pencils in the form of a thermoplastic sheath extruded aroundand directly bonded to a marking core. It is another object to provide amethod of the character described which is continuous in operation andproduces pencils having the physical characteristics of wood-sheathedpencils.

It is still another object to provide improved apparatus for using thecomposition of the invention, and to carry out the method of theinvention to produce improved thermoplastic resin sheathed pencils.

The composition of the invention which is extruded around the markingcore to form the sheath of the pencil consists essentially of, byweight, approximately 50 to 75% of a thermoplastic resin binder,approximately 20 to 40% of a fibrous filler, and approximately 0.5 to15% of a metallic soap. The ratio of binder to fibrous filler isapproximately 1.5 to 3.0. To furnish the desired fine, closed cellstructure for the sheath, the composition contains 0.3 to 2%, by weight,of moisture. Preferably, the moisture content is 0.8 to 1%.

By the method of the invention, pencils are made by formulating thesheath composition material, controlling the moisture content thereof,extruding it hot onto marking cores which are preheated to facilitatebonding to the sheath material, and then rapidly chilling the resultingextrudate comprising the marking core axially aligned within the sheath.After a final cooling, the extrudate is cut into the desired pencillengths. Subsequent to the formation of the sheath around the core butprior to the withdrawal of the extrudate from the die, the sheathsurface may be, and preferably is, coated with a pigmented thermoplasticcoating by extruding a coating composition onto it. Alternatively, theextrudate may be painted in the same manner as wood-sheathed pencils.

The apparatus of this invention comprises a crosshead and die assemblyadapted to extrude a softened or fused resin-based material around amarking core, means to feed the core material, in the form of abuttinglengths, into the crosshead and die assembly, means to preheat the corelengths prior to their introduction into the crosshead and die assembly,means to provide the sheath forming material as a fused resin-basedmaterial into the crosshead and die assembly, means to rapidly chill theextrudate sheath containing the core and then to further cool it, andmeans to withdraw the extrudate from the die. Means may also be providedfor extruding a resin coating over the extruded sheath before theextrudate leaves the die, and to cut the extrudate into predeterminedlengths.

The resulting pencil is characterized by good sharpenability, accuratealignment of the core within the sheath and tightly bonded thereto, asmooth pleasing surface, and completely adequate strength. Examinationof the sheath material shows it to be in the form of fine, closed,uniformly sized cells, believed to be the result of a small, butcritical amount of moisture present in the sheath forming material priorto extrusion. The size of these fine closed cells decreases radiallyfrom the core out to the pencil's surface, producing a concomitantincrease in density radially outward to provide a smooth exteriorsurface. It may be postulated that this in turn is due to the rapidchilling of the extrudate after it leaves the die nozzle.

These, and other objects and advantages of the invention will beapparent from the following detailed description taken in conjunctionwith the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram showing the steps in compounding the sheathmaterial and in the formation of a pencil using the sheath material;

FIG. 2 is a simplified, top plan view of apparatus suitable forprocessing a sheath composition in accordance with the invention, thisview further indicating an exemplary set of temperatures for one sheathcomposition at various zones of the apparatus;

FIG. 3 is a side elevation, partly cut away, of the marking core feedingmechanism;

FIG. 4 is a vertical cross section taken approximately in the plane ofline 4--4 of FIG. 3;

FIG. 5 is a top plan view of the core transport system;

FIG. 6 is a vertical cross section taken approximately in the plane ofline 6--6 of FIG. 5;

FIG. 7 is a side elevation, partly cut away and in cross section, of thecore preheater;

FIG. 8 is a vertical cross section of one embodiment of a crosshead anddie assembly and its relationship with the core preheater;

FIG. 9 is a cross section taken approximately in the plane of line 9--9of FIG. 8;

FIG. 10 is a cross section of another embodiment of a portion of the dieshowing a hexagonal-shaped die as an alternative to the circular die ofFIG. 9;

FIG. 11 is a cross section taken approximately in the plane of line11--11 of FIG. 10.

FIG. 12 is a vertical cross section of another embodiment of thecrosshead and die assembly which includes means to apply a coating tothe outside surface of the sheath;

FIG. 13 is a side elevation, partly cut away and in cross section, ofthe chilling means;

FIG. 14 is a cross section taken approximately in the planes of lines14--14 of FIG. 13;

FIG. 15 is a perspective view, partly in cross section, of the coolingmeans;

FIG. 16 is a view of the pulling means;

FIG. 17 illustrates a pencil made in accordance with this invention; and

FIG. 18 is an enlarged cross section of the pencil shown in FIG. 17.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described in greater detail with regard to thecomposition and formulation of the sheath material, the method of usingthe sheath material to form pencils, the apparatus used to carry out themethod, and the resulting article of manufacture.

COMPOSITION OF THE SHEATH MATERIAL

The sheath material consists essentially of a thermoplastic resinbinder, a fibrous filler and a metallic soap. Each of these essentialcomponents is believed to serve several purposes in creating a goodsheath.

The thermoplastic resin functions as a binder for the filler, to enhancethe extrudability of the sheath at relatively low temperatures, and tocontribute to the ease of fabrication to give a sharpenable,low-stretch, substantially rigid sheath.

Since no adhesive need be used, the resin binder is capable of providinga suitable bond to the marking core, and also preferably, the sheath iscapable of providing a base for the bonding thereto of a coatingmaterial. Thermoplastic, extrudable grade resins suitable as a componentof the sheath material include, but are not limited to,styreneacrylonitrile resins, acrylonitrile-butadiene-styrene resins,rubber-modified impact polystyrene resins, ethyl cellulose and thecellulose ester resins (e.g., cellulose acetate, cellulose butyrate andcellulose propionate), homopolymers and copolymers of the vinyl resinsand the like. A preferred thermoplastic resin is anacrylonitrile-butadiene-styrene resin having a Vicat softening point of212° F and a specific gravity of 1.05. In general, the Vicat softeningpoint of the resin is preferably above about 180° F.

The amount of the thermoplastic resin binder may range betweenapproximately 50 and 75% by composition weight, with approximately 65 to75% preferred.

The fibrous filler performs the multiple functions of serving as areinforcing filler to increase rigidity and flexural strength, enhancingthe sharpenability of the final pencil, and as the source of themoisture required in the formation of the fine cellular structureattained and responsible for the good sharpenability and low density.

Suitable fillers are preferably cellulosic in nature, but such fillersas micronized leather may also be used. Cellulosic fillers which may beused include, but are not limited to, such comminuted materials assoftwood flour, cotton linters, chopped or hammermilled rayons, hardwoodflours, walnut shell flour, peanut shell flour, pecan shell flour, sisalfibers, cotton flock, wood pulp and mixtures thereof. A preferred fillermaterial is a softwood flour sized such that 96 to 98% passes through a100-mesh screen, e.g., of a particle size of about 150 microns and less.Wood flours between 200 and 40-mesh size, e.g., between about 75 and 425microns may be used. The filler should be present in an amount rangingbetween approximately 20 to 40%, by weight, of the sheath composition.

The metallic soap functions as an extrusion aid to permit the use oflower than normal extrusion temperatures and acts as a lubricant to aidsharpenability. It appears to also contribute to the attainment ofuniformity of cell formation and to the resulting desired cell structurein general.

The term "metallic soap" is used herein in the sense defined on pp. 195,vol. 5, 1950 edition and 573, vol. 12, 1954 edition of the Kirk-OthmerEncyclopedia of Chemical Technology. Metallic soaps contain a metalother than sodium or potassium, and are for the most part alkaline earthor heavy-metal salts of monobasic carboxylic acids of the generalformula (RCOO)_(x) M, where M is a metal radical of valence state x andR is an organic radical containing at least 6-7 carbon atoms. Whilemetallic soaps are insoluble in water, the term as herein used includeslithium salts which are moderately soluble. Examples of metallic soapswhich are preferred are the stearates, oleates, palmitates.ricinoleates, and laurates of calcium, aluminum (mono-, di-, and tri-),magnesium, zinc, and lithium. Such metallic soaps are preferred becauseof their non-toxicity.

A preferred metallic soap is a micronized aluminum stearate having amelting point of between 150° and 155° C (about 300° F) and sized suchthat about 95% passes through a 200-mesh screen, e.g., of a particlesize of about 75 microns or less. For uniform distribution, the metallicsoap preferably is used in the form of a fine particulate material, andit must have a softening point below the temperature at which the sheathcomposition is extruded around the core.

The metallic soap is present in the sheath composition in a rangebetween about 0.5 and about 15%, by weight, with a preferred range beingbetween about 3 and 10% by weight.

To achieve the aforementioned desired criteria of sharpenability,adequate stiffness, good flexural or breaking strength and bondabilityto the marking core coupled with the essential ability to extrude thecomposition about the core, the binder and fibrous filler must bepresent in the binder/filler ratio of 1.5 to 3.0. To extrude acomposition having a binder/filler ratio of 1.5, it is desirable to usean amount of metallic soap at the higher end of the range indicatedabove, or approximately 15% by weight of the composition. It ispreferred however, that the composition contain a binder/filler ratiowhich is on the high side of the range, or approximately 2.8. Such acomposition provides a higher output rate, improved breaking strengthand a superior surface finish.

Though not essential, it is within the scope of the invention to addother ingredients including coloring material in the form of dyes orpigments, scenting agents such as those giving an odor of cedar, and thelike. These additional components are generally present in relativelysmall amounts, and are chosen to contribute any desired aestheticqualities to the finished pencil.

A significant aspect of this invention is that the sheath has a fine,controlled size, closed-cell structure. In the normal extrusion ofthermoplastic resins, it has always been considered essential to utilizea dry resin compound to avoid cell formation and to eliminate largeclosed voids in the extruded product. It has been found, however,contrary to this past practice, that if a small, but controlled amountof moisture is present in the sheath material as it is introduced intothe extruder, it is possible to utilize the moisture for the formationof the desired cellular structure in the finished extruded sheath. Theamount of moisture in the sheath composition is in the range of betweenabout 0.3 and about 2%, by weight, with the preferred range beingbetween about 0.8 and 1%. By controlling the moisture content of thesheath compositin within this fairly narow range and by using relativelylow extrusion temperatures and high extrusion pressures, it is possibleto control the cell size and distribution within the desired ranges.

Where the conditions are such that it is difficult to control thehumidity conditions, or to allow less rigid control of moisture, a smallamount of blowing agent may be included in the sheath composition.Examples of suitable blowing agents which may be used areazodicarbonamide 1,1' -azobisformamide, p, p' -oxybis (benzene sulfonylsemicarbazide), dinitrosopentamethylene tetramine, bis-benzenesulfonylhydrazide, asobisisobutyronitrile, and sodium bicarbonate. The blowingagent may be present in the amount of 0 to 2.0%, the preferred rangebeing 0.05 to 1.0% by weight of the total composition. If a blowingagent is used, the moisture content may be somewhat reduced withoutadversely affecting the uniform cellular structure of the extrudedproduct.

As hereinbefore indicated, one of the functions of the metallic soap isthat it acts as an extrusion aid. If desired, a small amount of anextrusion aid in addition to the metallic soap may be included in thesheath composition; for example, polyethylene, a hydrocarbon wax, afatty acid ester, a fatty acid amide, or a fatty acid. If an addedextrusion aid is included in the composition, it may be present in theamount of 0 to 5.0%, the preferred range being in the amount of 0.5 to2.5%, by weight, of the total composition.

Since the fibrous fillers normally contain moisture (e.g., approximately6% equilibrium moisture content in wood flour) the moisture ispreferably introduced as an integral part of the filler, and thenreduced to the desired level by heating. Alternatively to thecompounding of sheath composition as a dry material, it may be mixed asa slurry and the water removed in an appropriate manner to the desiredlevel.

In formulating the sheath composition, the fibrous filler and metallicsoap are dry blended at room temperature in conventional blendingequipment. Then this dry mixture, together with the thermoplastic binderare fed into a continuous-melt blender such as a Banbury or Farrelmixer. The blending temperature is maintained at a sufficiently highlevel to melt the resin binder without thermally degrading the filler orthe resin. Subsequent to mixing and cooling, the product is granulated,brought to the desired equilibrium moisture content by adding orremoving water and then sealed in an air-tight, moisture-impermeablecontainer until used.

If desired, the resin binder may be in the form of a latex, whereupon itis added to the fibrous filler and mixed until the filler is uniformlydistributed and thoroughly wetted out by the latex. Then the metallicsoap is added and thoroughly blended in. The resulting slurry or wetpaste is then dried in any suitable manner, such as by passing itseveral times through a two-roll mill heated to a temperature within therange specified for dry mixing to drive off excess water. This millingis continued until the composition forms a continuous band on one of thehot rolls. It is then removed and transferred to a heated blender forprocessing in the same manner as described for the relatively dryblends.

Sheath compositions were dry blended by the first described formulatingprocedure as follows:

    ______________________________________                                                     Percent by Weight of Component                                                Example Number                                                   ______________________________________                                        Component      1     2     3   4   5   6   7    8                             ______________________________________                                        Resin Binder                                                                  Acrylonitrile-butadiene-                                                                     70              70  65  57  54  51                             styrene resin (1)                                                             High-impact, rubber  70    70                                                 modified polystyrene (2)                                                      Fibrous Filler                                                                Wood Flour (100-mesh)                                                                        25    25        27  25  38  36  34                             Wood flour (40-mesh)       25                                                 Metallic Soap                                                                 Aluminum Stearate                                                                            5     5     5   3   10  5   10  15                             ______________________________________                                         (1) Sold as ABS 213 By Dow Chemical Company: Vicat softening point            212° F. specific gravity 1.05; melt flow rate 6.8 gm/10min.; melt      viscosity 6000 poises.                                                        (2) Sold as Styron 475B by Dow Chemical Company: Vicat softening point        206° F, specific gravity 1.05; melt flow rate 3.6 gm/10 min.; melt     viscosity 3400 poises.                                                   

(1) Sold as ABS 213 by Dow Chemical Company: Vicat softening point 212°F, specific gravity 1.05, melt flow rate 6.8 gm/10min.; melt viscosity6000 poises. (2) Sold as Styron 475B by Dow Chemical Company: Vicatsoftening point 206° F, specific gravity 1.05, melt flow rate 3.6gm/10min.; melt viscosity 3400 poises.

All of these sheath compositions were successfully used to make pencilsby the method and apparatus decribed below. When the sheath material isextruded to form the fine celled structure, it has a density rangingfrom 30 to 60 pounds per cubic foot.

METHOD OF FORMING PENCILS

FIG. 1 is a flow diagram illustrating the steps of the method of thisinvention. Of the steps shown in FIG. 1, at least three are believedsignificant. These are the controlling of the moisture content in thesheath composition, the preheating of the lead cores and the quickchilling of the extrudate immediately after withdrawal from the die.

The marking core is preheated to a temperature which approximates thatat which the sheath composition is introduced into the crosshead and dieassembly and is extruded around the core. This temperature is in turndependent upon the thermoplastic resin binder used, and is preferably atleast about 100° F above the Vicat softening point of the resin. Themarking core preferably is a lead; that is, a ceramic or vitrifiedelement made by the well-known technique of firing a mixture of clay andgraphite in the desired ratio. Since bonding of the sheath to the leadcore is achieved without the use of an adhesive, the lead core is madeso that it is free of any surface wax, the presence of which wouldinterfere with bonding.

The sheath composition with its controlled moisture content is heatedand forced under pressure through an extruder and caused to form asheath of a prescribed thickness around the preheated core positionedalong the axis of the sheath. The extrudate as it is withdrawn from thenozzle of the die is immediately chilled by contacting its entiresurface with expanding, compressed air which strikes the extrudatesurface in the form of a multiplicity of gas streams. These air streamsare directed into the chilling zone toward the outlet end of the chillerto form an acute angle with the extrudate surface. The extrudate duringthis chilling step is supported by the die nozzle and by a cooling meanswhich follows the chilling zone so that its entire surface is exposed tothe compressed air. Subsequent to this chilling step, the extrudate issubjected to further cooling by indirect (and if desired direct) heatexchange with a fluid coolant until it reaches substantially ambienttemperature throughout. Withdrawal of the extrudate from the die nozzleand through the chilling and cooling steps is accomplished by a degreeof positive pulling which may be regulated automatically to ensure auniform cross section in the extrudate. As a final step, the extrudateis cut into predetermined pencil lengths which may then be stamped withany desired indicia and equipped with an erasure assembly as is wellknown in the pencil making art.

The extrudate may be coated with a pigmented coating composition bycoextruding a thermoplastic resin coating containing pigment in the formof a thin skin on it prior to its withdrawal from the die. The resincoating composition is heated above its softening point for coextrusionand is applied subsequent to the formation of the sheath. Suitableresins for coating include, but are not limited to, polystyrene,rubber-modified impact polystyrene resins,acrylonitrile-butadiene-styrene resins, styrene-acrylonitrile resins,acrylic resins, polyamide resins, cellulose-based resins, vinyl resinsand the like. The coating resin must of course be one which has aworking temperature which is within the temperature range at which thesheath is formed and be capable of forming a permanent bond with thesheath surface.

FIG. 2, which is discussed in detail below, gives exemplary processingtemperatures when using the composition of Example 1 as the sheathmaterial and a colored styrene resin as the coating material. It will beseen that whereas the acrylonitrile butadiene-styrene resin used as thebinder in the sheath composition is normally extruded at about 425° F,the sheath material of this invention containing the relatively highpercentage of metallic soap is extruded at about 325° F.

APPARATUS FOR FORMING PENCILS

The apparatus of this invention is shown in a simplified top plan viewin FIG. 2. Details of the apparatus components are illustrated in FIGS.3-16. As shown in FIG. 2, the lengths of the marking core 10 aresupplied to and fed from a core feeder 11, moved along in abuttingrelationship by a core transport mechanism 12 into a core preheater 13.The granulated sheath composition material is fed from a bin 14 into anextruder 15 through an inlet adapter line 16 and into a crosshead 17. Itwill be appreciated by those skilled in the art of extrudingthermoplastic materials that suitable heating means (not shown) must beprovided to establish the desired heating zones. Exemplary heaters areshown for the crosshead 17 in FIGS. 8 and 12 described below. Affixed tothe end of a die 18 and thermally isolating it from chilling means 19 isa radiation shield 20, typically formed of a thermally insulatingmaterial such as asbestos board. Compressed air is provided to the chillmeans 19 through conduits 21, one of which is illustrated. The extrudate22 is withdrawn from the die 18 by means of a puller 23, passing throughthe chilling means 19, through a cooling means 24, which is designatedto effect indirect heat exchange between the extrudate and a coolant(e.g., tap water). The coolant is introduced into the cooling means 24through an inlet line 25 and withdrawn through a discharge line 26. Theextrudate is cut into pencil lengths 27 by a cutter 28. If the extrudateis to be coated by coextruding a thermoplastic resin skin coating on itssurface, the coating resin is fed from supply means 29 into a coatingextruder 30, through a die inlet adapter conduit 31 into the die 18. Asin the case of the sheath extrusion means, suitable heating means (notshown) are provided for the coating extruder. If not so coated, then thecut pencil lengths are coated in the same way as wood-sheathed pencilsare coated.

The core feeder 11 and core transport means 12 are shown in FIGS. 3 and4. The cores are supplied in predetermined lengths 10 and are fed intothe preheater, crosshead and die in abutting relationship to form, inessence, an endless or continuous, aligned core. The core lengths 10 arefed singly through a vertical feeding passage 40 defined between twoplates 41 and 42 closed at their ends by end members such as 43 andtapered at the delivery end. A core supply bin 44 provides the corelengths to the feeding passage 40.

The marking core lengths 10 are moved forward out of the feeder onto anendless V-belt 45 which engages a forward pulley system 46 and an afterpulley system 47. These pulley systems are mounted on shafts 48 and 49,respectively, rotating in forward pillow blocks 50 and after pillowblocks 51. The after pulley system 47 is driven by motor 52 throughdrive pulleys 53 and 54 which are mechanically connected by belt 55. Thepillow blocks 50 and 51 and the motor support 56 are all mounted on abase plate 57.

As the cores 10 are delivered to the V-belt, they are initiallyaccurately aligned and maintained in alignment for delivery into thecrosshead. Alignment is established and maintained within the feederfirst through the alignment of belt 45 and then through alignment of thecores on the belt. As shown in FIGS. 3 and 4, alignment of the belt ismaintained by causing it to travel between two aligning strips 60 and 61affixed to a belt support plate 62. The inboard sides of these aligningstrips are bent upwardly at an angle conforming to the contour of thesides of the belt to form belt guides 63 and 64 which are spaced todefine a passage just large enough to permit the belt to travel throughthe passage with a minimum amount of friction. Alignment of the cores onthe belt is achieved through the use of core aligning strips 65 and 66,the inboard sides 67 and 68 of which are bent downwardly to form anelongated lead core passage 69 directly beneath lead core feed passage40. This elongated passage 60 is bounded on its upper side by the exitof passage 40 and on its lower side by the surface of belt 45. Thealigning strips 65 and 66 are mounted on angled support strips 70 and 71which in turn are adjustably mounted on lower angled supports 72 and 73,affixed to belt support plate 62, and to upper angled supports 74 and75. These latter, upper supports 74 and 75 in turn form the means bywhich the two plates 41 and 42 defining passage 40 are supported,through angled strips 76 and 77, above the belt. This entire supportsystem is adjustable with regard to distance from the core axis as wellas to height above the belt support plate 62.

The belt support plate 62, along with the support assembly affixed toit, is adjustably positioned above base plate 57 by means of forwardangled support plates 79 and 81 and after angled support plate 78 and 80which are apertured to permit the passage of belt 45.

As previously stated, it is necessary to advance the core lengths 10 inabutting relationship. The advancement of the cores is accomplished bymeans of a series of pusher feet 89 mounted at equally spaced intervalsalong the center line of the belt, the intervals being equivalent to thelength of the individual lead cores plus a small clearance. These pusherfeet also control the discharge of a core length onto the belt, for aslong as one of these pusher feet is traveling under the exit of passage40 in the feeder, it is not possible for the core at the exit of passage40 to fall completely onto the belt. As soon, however, as a pusher foot89 (e.g., the one shown in FIG. 3) passes beyond the forward end of thepassage exit, the next core falls into place and a pusher foot makescontact with its after end. A positive force is required to move thecores forward to overcome the resistance provided in the crosshead asthe extruded sheath composition contacts the cores. This positive forceis provided by the pusher feet 89. The gap between consecutive corelengths is eliminated by providing the motor 52 with a slip clutchmechanism which allows the belt 45 to move faster when it is notactually pushing a core length. Thus each core length "catches up" withthe one ahead of it which, as explained, experiences a backward thrust.

The core transport mechanism 12 between the feeder 11 and preheater 13is shown in FIGS. 3, 5 and 6. The angled support strips 70 and 71 extendbeyond the belt 45, while the core alignment strips 65 and 66 extend tothat point where the belt contacts the pulley 46. The angled supportstrips 70 and 71, beyond the effective length of belt 45 serve tosupport a trough 90 (FIG. 6) which is aligned with belt 45 and designedto provide a passage 91 for the core 10. Passage 91 is enclosed on topby a solid metal bar 92 which is cut to fit the sides of the trough andadapted to sit in it without touching the lead cores. This bar preventsany buckling of the core lengths where they abut. The trough 90 and theentry end of the core preheater are joined by a cover plate 93 servingin the same role as the solid bar 92. At this point, the core isintroduced into the preheater.

The core preheater 13, as shown in FIGS. 7 and 8, is formed in foursections: a small diameter entry section 100, a larger diameter heatersection 101, an adapter section 102 and a threaded section 103. The topof the entry section 100 is partially cut away to provide a flat surface104 for engagement with cover plate 93 of the core transport system. Theheater section comprises a cylindrical thermal mass of metal 105(typically formed of steel or copper) and a band resistance heater 106in thermal contact with substantially all of its surface. A thermocouplewell 107 is located in the thermal mass 105. It is, of course, withinthe scope of this invention to use other types of heaters and othertypes of temperature measuring devices if desired. A passage 108, havinga diameter slightly greater than the diameter of the core, extendsthroughout the length of the preheater.

One embodiment of the crosshead and die assembly is shown in crosssection in FIG. 8. In keeping with well known crosshead design practice,the crosshead used in this apparatus comprises a crosshead body formedof a male section 110 and a female section 111. The male section has athreaded well 112 adapted to be connected to the threaded conduitadapter 16 of the sheath material extruder (FIG. 2) to provide thenecessary communication with the annular extrusion passage 113 definedbetween the male and female sections. The female section is designed toprovide a circumferential ridge 114, the purpose of which is to ensureuniform distribution of the thermoplastic sheath composition material115 (FIG. 9) around and throughout passage 113. A cone 116 is attached,such as by screw means, to the preheater extending through the femalesection. The cone extends into the die 117, forming therewith a passage118 which is a continuation of passage 113. The cone 116 has a corepassage 119 aligned with passage 108 in the preheater. This passage 119and the conical passage 118 converge in the after end of the die at thepoint where the sheath material is extruded around and bonded to thecore forming the extrudate 22. The crosshead sections and the die,including the crosshead clamp 120, are assembled by suitable means suchas screws, as illustrated in FIG. 8. Two band resistance heaters 121 areaffixed to the outer surface of crosshead 17 and another band heater 122is affixed to the outer surface of die 117. The apertured radiationshield 20 interposed between the heated die section 18 and the chillingmeans 19 is attached to the end of the die.

As shown in FIGS. 8 and 9, the extrudate 22 (core 10 surrounded bysheath material 115) is circular in cross section. It is also within thescope of this invention to form the extrudate in any other desirablecross section, e.g., hexagonal. This latter configuration is illustratedin FIGS. 10 and 11. In FIG. 10, the die opening 123 is shown without thesheath material while in FIG. 11 the sheath 115 is shown around the core10.

FIG. 12 illustrates another embodiment of the crosshead and dieassembly, this embodiment being adapted to coextrude a skin coatingaround the sheath. The main crosshead is the same as shown in FIG. 8.The die assembly is modified to have an additional crosshead and dieattached thereto, as by threading it onto the end of die 117. Thissecond crosshead and die comprise a threaded adapter piece 125 which hasmachined in its face 126 a shallow circular well 127 bordered by asomewhat deeper ring 128, the well and ring forming with die block 129 aportion of the passage through which the thermoplastic resin coatingmaterial is introduced into the die. The circular well 127 is located sothat its center does not coincide with the aperture in adapter 125, thusensuring uniform distribution of the resin prior to and during itscontacting the exterior surface of the sheath. The die block 129 has athreaded well 130 which is adapted for attachment to adapter conduit 31of the coating extruder 30 (FIG. 2). This well is in communication withring 128 through passage 131. The die opening in die block is sized topermit the formation of a coating 132 of a predetermined thickness to becoextruded around the sheath to form the extrudate. In this embodimentof the invention, the radiation shield 20 is bolted to die block 129.

One means for effecting the required rapid chilling is illustrated inFIGS. 13 and 14. It provides means for contacting the entire surface ofthe extrudate with cooled expanded high-pressure air to give the hotextrudate a smooth surface and to impart to the sheath a radial densitygradient. This chilling means comprises three concentric lengths oftubing, typically about three feet long held in spaced relationship bymeans of annular spacing rings 140 and 141. The outer tubing 142 has twoair inlet ports 143 and 144 opposite each other and located at about thecenter of its length. These are adapted to receive compressed air fromair inlet lines 21 (FIG. 2). The first inner tubing 145 defines anannular air flow passage 146 with the outer tubing, and it is perforatedto provide a number of fluid ports 147 communicating with the innermostannular passage 148 defined between the first inner tubing 145 and asecond inner tubing 149. The second inner tubing has a number of fluidports 150 positioned to be staggered from the fluid ports 147. The fluidports 150 are cut at an angle to cause the air entering the chillingzone 151 (defined within the second inner tubing) in the form of amultiplicity of streams directed into the chilling zone toward theoutlet end of the chiller at an acute angle with the extrudate surfaceas indicated by the arrows in FIG. 13. The annular passages 146 and 148are sealed at the ends by the annular spacing rings 140 and 141. Thechiller is conveniently supported by two pairs of legs 152 and 153.

From the chiller 19, the extrudate passes to the cooling means 24 ofextended length, typically about forty feet, adapted to effect indirectheat exchange with a fluid coolant, such as tap water. As shown in FIG.15, the cooling means comprises two end-sealed lengths of copper tubing160 and 161 soldered together and having a fillet 162 formed of a goodheat conducting material contoured to contact a part of the surface ofthe extrudate 22. The coolant is delivered to the forward end of each ofthe tubings through inlet conduits 25 and 25a and withdrawn at the afterend through discharge conduits 26 and 26a. Final additional cooling, ifdesired, may be effected by directly contacting the extrudate surfacewith a fluid coolant, e.g., one or more fine streams of water or air.

It is necessary to exert a pulling force on the extrudate to ensure aneven cross section throughout. The pulling force may be developed by anumber of different types of equipment known in the art, the puller 23shown in FIG. 16 comprising two hard rubber rolls 165 and 166 mounted onshafts 167 and 168. One of the shafts is driven by any suitable meansnot shown. It may be desirable to automatically control the speed of theshaft driving means to control and regulate the extrudate diameter.

The cutter 28 (FIG. 2) may be chosen from commercially availabledevices; and it is preferably one that advances as it cuts the forwardmoving extrudate so that it forms a perfectly square cut. Such cuttersare known.

COMPLETED ARTICLE OF MANUFACTURE

The cut lengths 27 of the extrudate may have erasers 175 attachedthrough metal sleeves or ferrules 176 (FIG. 17) to be formed intopencils. The pencils made by the method and apparatus of this inventionare readily sharpenable in a mechanical pencil sharpener as well as witha penknife. The outer surface, whether coated or not, is smooth and canbe printed on using the same techniques which are presently used forall-wood sheathed pencils. The actual sheath structure of the pencils isunique in that it may be seen in cross section to be formed of finelysized, closed cells. This structure is illustrated in a diagrammatic,not-to-scale drawing in FIG. 18. The cells 180 nearer the center aretypically about 70 microns in diameter, while those cells 181 around theouter edge are substantially closed or not greater than about 10 micronsin diameter. The larger cells may have diameters up to about 100microns. The cell diameter gradient decreases radially outward tofurnish a density gradient which increases radially outward. It may bepostulated that the denser outer surface 181 providing the smoothsurface of the extrudate, together with the internal cell structure, isdue to the controlled amount of moisture, to the use of the metallicsoap, and to the rapid chilling of the extrudate as it exits from thedie. As shown, the extrudate is provided with the skin coating 132.

It is believed that the advantages and improved results afforded by thesheath composition, method and apparatus for making pencils, and theresultant product of invention will be apparent from the foregoingdetailed description. Various modifications and changes may be madewithout departing from the spirit and scope of the invention as setforth in the following claims.

We claim:
 1. A pencil comprising a marking core and a sheath bonded tothe core, the sheath comprising an extrusion grade thermoplastic resinbinder having a Vicat softening point above about 180° F., a comminutedcellulosic fiber filler, and a substantially water-insoluble metallicsoap, the binder/filler ratio being approximately 1.5 to 3.0, the sheathbeing provided with closed cells having a maximum diameter ofapproximately 100 microns, the size of the cells decreasing radiallyoutward from the core to provide a substantially smooth outer surface onthe sheath.
 2. A pencil comprising a marking core and a sheath bonded tothe core, the sheath consisting essentially of, by weight, approximately50 to 75% of an extrusion grade thermoplastic resin binder having aVicat softening point above about 180° F., approximately 20 to 40% offibrous filler particles selected from the group consisting of celluloseand leather, the binder/filler ratio being approximately 1.5 to 3.0 andapproximately 0.5 to 15% of a substantially water-insoluble metallicsoap, the sheath being provided with closed cells and having asubstantially smooth outer surface.
 3. A pencil according to claim 2,wherein the thermoplastic resin binder is selected from the groupconsisting of styrene-acrylonitrile-resins,acrylonitrile-butadiene-styrene resins, rubber-modified impactpolystyrene resins, ethyl cellulose, cellulose ester resins,homopolymers and copolymers of vinyl resins, and mixtures thereof in theamount of approximately 65 to 70%; wherein the fibrous filler iscomminuted cellulosic material in the amount of approximately 25%; andwherein the metallic soap is in the form of particles, at least 95% ofwhich are no greater than 75 microns in diameter, the metallic soapbeing present in the amount of approximately 3 to 10%.
 4. A pencilaccording to claim 3 wherein the thermoplastic resin binder isapproximately 70% of an acrylonitrile-butadiene-styrene resin; whereinthe fibrous filler is approximately 25% wood flour; and wherein themetallic soap is aluminum stearate and is present in the amount ofapproximately 5%.